1. Field of the Invention
This invention relates generally to optical data storage systems and more particularly to pulse width modulation recording in such systems.
2. Description of the Prior Art
Pulse position modulation (PPM) and pulse width modulation (PWM) are two general ways to record information on data storage media. PPM records information as the distance between the centers of the marks on the medium. PWM records information as the distance between the transitions of the marks. A transition is either the beginning (leading) or end (trailing) edge of a mark.
PWM recording is preferred to PPM recording because PWM recording is able to store more information in the same amount of space on the medium. In other words, the data storage density can be greatly increased by using PWM recording. However, PWM recording is more difficult to implement because the transition edges must be precisely positioned and written with sharp boundaries to ensure accurate recording. Unwanted variation in laser power, magnetic field strength, operating temperature, the number of times the media has been written and erased (cycling), etc., can cause the exact position of these transitions to change. In addition to these problems, thermal buildup within the medium during the writing process can cause great distortions in the placement of the mark edge. U.S. Pat. No. 4,928,187 issued May 22, 1990 discusses this problem.
One solution to this problem is to use direct overwrite. In a direct overwrite system, the laser beam is left on continuously and the magnetic field is modulated in order to record data. U.S. Pat. No. 4,872,078 issued Oct. 3, 1989, shows one such system.
Another solution to the thermal problem is to adjust the laser pulse duration in order to achieve the correct mark length. Some of these systems use a series of highly pulsed laser beams to write contiguous or overlapping circular marks on the medium in order to form one long mark. Examples of these systems include U.S. Pat. No. 4,473,829 issued Sep. 25, 1984; U.S. Pat. No. 4,488,277 issued Dec. 11, 1984; U.S. Pat. No. 4,734,900 issued Mar. 29, 1988; U.S. Pat. No. 4,873,680 issued Oct. 10, 1989; U.S. Pat. No. 4,998,237 issued Mar. 5, 1991; Japanese patent application 03-35425 published Feb. 15, 1991; Japanese patent application 02-208834 published Aug. 20, 1990; Japanese patent application 01-155522 published Jun. 19, 1989; and Japanese patent application 63-263632 published Oct. 31, 1980. However, these systems still experience some thermal buildup with the resulting inaccuracy in the placement of the transition edge and blurring of the boundary.
What is really needed is a relatively simple system and method to record mark transitions precisely without experiencing thermal buildup.